A field effect transistor having a nitride heterojunction easily performs high-voltage and high-temperature operation in the microwave band or the higher frequency band, and can be used for mobile radio base stations, radar equipment, etc.
When an AlGaN layer is provided on a GaN layer serving as a channel layer, positive charges are generated at the heterojunction interface and negative polarization charges are generated at the AlGaN surface.
When large signal operation of a field effect transistor having such a structure is performed at microwave frequency, the current collapse phenomenon may occur in which the maximum drain current is decreased as compared to in direct current operation. Furthermore, in large signal operation, since the peak voltage of the gate electrode is higher than the direct current voltage, electric field concentration may occur at the semiconductor surface near the gate electrode and high-voltage operation may be suppressed.